Traffic generated by time critical applications has typically been transported in proprietary dedicated networks such as Cobranet, ISO/IEC 62439, 61158 or 61784, and Avionics Full-Duplex Switched Ethernet (AFDX). Recent developments attempt to move time critical traffic to packet networks, such as Layer 2 bridged networks, that also handle other traffic with less stringent requirements. As a result of these developments, commodity Ethernet with, for example, Audio Video Bridging (AVB) is being introduced and used in industrial automation, professional audio processing and vehicular networking.
Another area in which the introduction of Layer 2 bridged networks is being considered is the fronthaul domain of mobile communication networks. The fronthaul domain connects a mobile operator's radio equipment(s) and radio equipment controller(s). This connection has technically challenging latency and Packet Delay Variation (PDV) requirements as defined, for example, by the Common Public Radio Interface (CPRI). In view of these requirements, the fronthaul domain is presently not provided by bridged networks but by dedicated lambda connections in optical networks. On the other hand, the benefits of sharing network resources by time critical traffic and other traffic would also be an interesting option for the fronthaul domain in case latency and PDV can be kept low.
One approach to reduce latency and PDV for time critical traffic is the classification of different traffic types such that time critical traffic can be transported in the communication network with a higher priority than other traffic. In the Ethernet standard, for example, eight traffic classes are defined to permit a prioritized handling of individual traffic classes.
Newer developments in the Ethernet standard permit the declaration of one of the eight traffic classes as express traffic class that receives particular treatment in an Ethernet bridge to selectively reduce packet delay for the associated traffic. For example, the draft IEEE standards 802.1Qbu “Frame Preemption” and 802.3br “Interspersing Express Traffic” suggest suspending transmission of less time critical (“preemptible”) traffic to the benefit of time critical express traffic. The draft IEEE standard 802.1Qbv “Enhancements for Scheduled Traffic”, on the other hand, targets at a packet delay reduction by draining transmission queues with respect to a well-defined timing. All of these IEEE draft standards relate to the egress, or output, port of a network bridge.
The terms “egress” and “output” as well as “ingress” and “input”, respectively, are used interchangeably herein. Also the terms “frame” and “packet” are used interchangeably herein.
It has been found that despite the various approaches to reduce delay and PDV in communication networks, an individual packet can still suffer from significant delay, and a packet flow from considerable PDV. As an example, the race of two simultaneously received express traffic packets of different packet flows for service at network element ports can result in a significant queuing delay for the packet losing the race as they share the same resources (namely the ones dedicated to express traffic). In addition to an increase of the overall packet delay, the losing packet flow additionally suffers from PDV accumulation as the losing packet flow in essence “inherits” the PDV of the winning packet flow.
The declaration of express traffic per se can therefore not guarantee a low delay and a low PDV for each and every express traffic packet flow. For example, frame preemption cannot help solving the race condition among time critical packet flows that all belong to the express traffic class. However, if racing situations for such packet flows cannot be controlled, then additional buffering is needed to reduce the PDV. On the other hand, such additional buffering will further increase the delay experienced by the buffered packets. Similar problems occur in other situations in which different packet flows of the same traffic handling class need to be processed in a communication network.
For the above reasons, applications that are particularly time critical still have to rely on proprietary dedicated networks. This is an unsatisfactory situation because Layer 2 Ethernet bridged networks and similar networks are readily available.